Solar cells, of course, are semiconductor devices that have been the subject of substantial developmental effort over the years, including among many other aspects, improving their conversion efficiency and improving the methods and apparatus for producing solar cells at lower cost. These efforts have only been enhanced in recent years, with great concern and emphasis on attempting to progress toward parity in the cost of electricity generation by solar cells with traditional sources of electric power generation.
The use of silicon-based semiconductor material for solar cells has an extended history. However, the material and manufacturing costs of such solar cells are relatively high. Therefore, it is very difficult to achieve parity in electric power generation using silicon-based solar cells, in comparison to costs related to traditional sources of electrical energy. One important approach to overcoming these difficulties has been the effort directed to develop low cost, thin film solar cells. These efforts include concern with the process of making large area thin film solar cells, using amorphous and polycrystalline semiconductor material, by methods that are more cost effective and have increased material utilization. More specifically, the manufacturing of large area thin film solar cells, with a required conversion efficiency, in a cost effective way, has proven an extremely great challenge.
In manufacturing thin film solar cells, various layers of electronically active material are provided on a base (of one or more layers) of choice, to achieve a desired solar cell structure. One example of an electronically active layer that is often used is a zinc oxide (ZnO) layer that is generally transparent to the solar radiation of concern and that is also electronically conductive. The challenge for a layer such as this, as for a number of other formed layers, is the need to control its composition, its electronic and optical properties and its uniformity, over a large area substrate, while maintaining a significant rate of production.
Despite the desire for increased rates of production, due to the other concerns and problem areas, the choice of methods and systems for forming the electronically active, thin film solar cell layers, emphasize batch processing—i.e., production of one or a number of units in an apparatus, removing them, and replacing them with the next unit or group of units.
For the fabrication of thin film semiconductor devices, both physical vapor deposition (commonly abbreviated “PVD”) techniques, and chemical vapor deposition (common abbreviated “CVD”) techniques, have been employed. In regard to physical vapor deposition, prior, known methods employ Rf (radio frequency) magnetron sputtering in a continuous processing mode to form ZnO material layers—with units, one after the other, moving continuously along a sputter deposition path. However, the ceramic ZnO material targets used in this sputtering process are quite expensive, and the deposition rates for the ZnO material layers are quite low. Thus, to achieve desired production rates, typically, a large number of ZnO material targets are required, making the sputtering system relatively large and expensive.
Turning to the technique of chemical vapor deposition, prior, known efforts have been directed at devices that provide an enhanced area of distribution of chemical vapor for deposition. This, of course, thus, is directed to enhanced area solar cells. For example, for applying two separate material vapors, e.g., separately carried by a gas, such devices would typically provide separate conduit structures for each, with one supplying a set of generally parallel conduit structures having openings along their lengths, and the other supplying another such set that alternates in position with the conduits of the first set. Rather than alternate generally parallel conduits, a variation employs one set of conduits extending outwardly from a central area and another alternating set extending outwardly from that area. That variation, thus, is in the nature of a “sunburst” array. Each of these has been of limited usefulness.